Selling your old iPhone? Do it now

The trade-in value of old iPhones will plummet before Apple announces its newest version in September.

That old iPhone sitting in the bottom of your sock drawer could be worth $300 -- but only if you act now.

Old iPhones hold their worth better than any other smartphone on the market. But iPhones' resale value takes a nosedive right before Apple announces a new version of its smartphone -- and Apple(AAPL) is expected to do just that on Sept. 10.

Apple is widely expected to introduce a gold-colored iPhone, a fingerprint sensor and a new low-cost "iPhone C." No matter what it ultimately will look like, the new iPhone will send Apple fans running to the store and searching for a way to get their old phone out of their hands.

But waiting until the actual announcement means you are already late to the game.

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Gazelle, an online trade-in service that will pay you cash for sending in an old gadget, received one iPhone every five seconds last week. -- a 70% increase over the previous week. Gazelle currently pays $300 for a 16 gigabyte AT&T(T) iPhone 5 in good condition. You can even lock in that quote now and send in your phone up to 30 days later.

But that price will continue to drop until weeks after the announcement is made.

"There is a predictable 15% to 20% value decline seen across all older iPhone models in the six-week period surrounding the new iPhone launch," said Jeff Trachsel, the chief marketing officer of a similar buyback service called NextWorth.

The same iPhone 5 that NextWorth would have paid $314 for at the end of last week has already dropped $6 in value this week, as last year's iPhone models have flooded the resale market over the past few days.

This year, there is even more urgency to sell early than in previous years. iPhone trade-in traffic at Gazelle soared 30% last week when compared to the year before.

"Consumers are becoming very tech savvy and more aware that their old device is actually worth something," said Anthony Scarsella, the chief gadget officer at Gazelle.

About half of smartphone owners don't even think about trading in their device before stashing it in the bottom of a drawer or closet, but that number is decreasing, said Scarsella.

There are more ways to trade in your iPhone now than ever. A repair company called iCracked will even buy your phone off you if the screen shatters. You can try to sell your iPhone directly on eBay(EBAY) or Amazon(AMZN), or use an online service like Glyde, an e-commerce site similar to eBay that targets consumers looking to sell tech gadgets. Carriers are also increasingly allowing customers to trade in their phones, and retail giants Best Buy(BBY) and Target(PBCFX) have recycling programs.

Smartphones -- and iPhones in particular -- create a kind of perfect storm for a secondary market.

Smartphone subsidies allow consumers to sell their older model after two years when their contract is up and get almost what they paid for it, said Trachsel. Plus, Apple's much anticipated launch announcements add to the hype.

"There's a group of people out there, myself included, that want the latest and greatest of whatever Apple does," he said.